1. Field of the Invention
The present invention generally relates to frequency converted laser sources and, more specifically, to frequency converted laser sources that include, inter alia, a semiconductor laser optically coupled to a second harmonic generation (SHG) crystal, or another type of wavelength conversion device, and methods for operating the same.
2. Technical Background
Short wavelength light sources can be formed by combining a semiconductor laser, such as an infrared or near-infrared distributed feedback (DFB) laser, distributed Bragg reflector (DBR) laser, or Fabry-Perot laser, with a wavelength conversion device, such as a second or higher order harmonic generation crystal. Typically, the wavelength conversion device is used to generate higher harmonic waves of the fundamental beam of the semiconductor laser. To do so, the lasing wavelength of the semiconductor laser is preferably tuned to the spectral center of the wavelength conversion device (e.g., the semiconductor laser is tuned for phase-matched operation with the wavelength conversion device) and the output beam of the laser is preferably aligned with a waveguide portion at the input facet of the wavelength conversion device to produce a frequency converted output beam from the wavelength conversion device. This arrangement may be referred to as a frequency converted laser source.
Waveguide optical mode field diameters of typical wavelength conversion devices, such as MgO-doped periodically poled lithium niobate (PPLN) second harmonic generation crystals, may be on the order of several microns. As a result of these dimensions, energy densities in the wavelength conversion device may be on the order of several MW/cm2 at optical power levels as low as 100 mW. The present inventors have found that long term generation of a frequency converted output beam at such high energy densities may lead to loss of transmission power of the fundamental beam of the semiconductor laser in the wavelength conversion device and a loss of power of the frequency converted output beam emitted from the wavelength conversion device. This phenomenon, which may be referred to as photo-darkening or photo-degradation, degrades the performance of the frequency converted laser source. Moreover, the performance loss is not significantly recovered following discontinuation of phase-matched operation of the semiconductor laser with the wavelength conversion device and/or extended periods of storing the wavelength conversion device in a “dark” condition.
Accordingly, alternative methods for operating frequency converted laser sources to mitigate the effects of photo-degradation are needed.